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Dr. Raymond Sandarin

Why Test inks Cannot Tell the Full Truth About Surface Free Energy

Adhesive bonding has been the tool of choice for connecting metals, plastics, or other materials of interest. Extensive pre-treatment as cleaning, surface roughening, or plasma activation are applied to these materials prior to the gluing process in order to improve the wettability of glue to the surfaces of a material. To monitor the efficiency of the pre-treatment, the surface-free energy (SFE) of the substrate is typically measured. In many cases, dyne inks are used to determine the total SFE following the assumption that a. surface having a SFE value above a. certain threshold is already sufficiently pre-treated for the following adhesive bonding.

It is well known, however, that the SFE is more than one single value and its distribution into polar and disperse constituents is essential if wetting and long term adhesion shall be characterized. In contrast to dyne inks, contact-angle measurements determine the polar and disperse contributions to the SFE. In a. thorough experimental study, we determined the SFE of various materials using different types of dyne inks and contact-angle measurements. The investigated materials range from polymers, glassware, silicon wafer, mica, and graphene, to metals like aluminum. In addition, we tested some materials before and after plasma treatment. Results obtained with test inks and contact-angle measurements were compared to illustrate advantages and drawbacks of either teclmique. We explicitly explain why for some materials the test inks and contact-angle measurements yield different results. For that purpose, we determined polar and disperse parts of test inks. Finally, we show how contact angle measurements on solid and liquid glue provide important information about the work of adhesion and interfacial tension determining short-teen bonding and long-tenn adhesion, respectively.